Field of the Invention
The present invention relates to a method of determining whether or not a protective film has a desired thickness, the protective film being formed on the front side of a workpiece such as a semiconductor wafer in dividing the workpiece by laser processing.
Description of the Related Art
As a method of dividing a workpiece such as a semiconductor wafer along streets (division lines), there is a method of laser-processing the workpiece along the streets to thereby divide the workpiece (see Japanese Patent Laid-open No. 1994-120334, for example). In the laser processing method described in Japanese Patent Laid-open No. 1994-120334, a laser beam is applied to the semiconductor wafer along the streets to generate thermal energy on the semiconductor wafer in an area corresponding to the streets, thereby continuously processing the semiconductor wafer in this area irradiated with the laser beam. There is a case that the thermal energy may be concentrated in this laser irradiated area on the semiconductor wafer to cause the generation of debris (processing dust). Accordingly, there is a problem such that this debris may adhere to bonding pads or the like on large scale integrations (LSIs) formed on the semiconductor wafer, causing a reduction in quality of semiconductor chips.
To solve this problem, the present applicant has proposed a laser processing method including the steps of forming a water-soluble protective film on the front side of a semiconductor wafer and next applying a laser beam through this protective film to the semiconductor wafer (see Japanese Patent Laid-open No. 2004-322168, for example). In the laser processing method described in Japanese Patent Laid-open No. 2004-322168, the semiconductor wafer is processed through the protective film, so that the debris scattered in laser processing can be made to adhere to the protective film. Thereafter, in a cleaning step, the protective film is removed together with the debris. Accordingly, the adhesion of the debris to the front side of the semiconductor wafer can be suppressed to prevent a reduction in quality of the semiconductor chips.